Video encoding

Abstract

The invention relates to a video encoder ( 201 ) for encoding a video signal. The video encoder comprises a segmentation processor ( 207 ) which divides the picture into picture regions. Preferably, picture regions having a high degree of flatness or uniformity are determined in this way. A characteristics processor ( 209 ) determine a spatial frequency characteristic for each picture region, and a coding controller ( 211 ) selects an encoding block size, such as a prediction block size for motion estimation, in response to the spatial frequency characteristic. An encode processor ( 213 ) encodes the picture using the selected encoding block size. Specifically, increasing block sizes are selected for increasing degrees of uniformity or flatness indicated by the spatial frequency characteristic. Thereby, an increasing proportion of high frequency components and a consistent choice of encoding block sizes are maintained, and thus the coding artefacts from many encoders having variable prediction block sizes is reduced. The invention is particularly suitable for H.264 and similar encoders.

1 . A video encoder ( 201 ) for encoding a video signal comprising: 
 means ( 207 ,  209 ) for determining a picture region having a spatial frequency characteristic;    means ( 211 ) for setting an encoding block size for the picture region in response to the spatial frequency characteristic; and    means ( 213 ) for encoding the video signal using the encoding block size for the picture region.    
     
     
         2 . A video encoder ( 201 ) as claimed in  claim 1  wherein the encoding block size is a motion estimation block size.  
     
     
         3 . A video encoder ( 201 ) as claimed in  claim 1  wherein the means ( 207 ,  209 ) for determining the picture region is operable to determine the picture region as a group of pixels for which the spatial frequency characteristic meets a spatial frequency criterion.  
     
     
         4 . A video encoder ( 201 ) as claimed in  claim 3  wherein the spatial frequency criterion is that a spatial frequency distribution comprises an energy concentration above an energy threshold for spatial frequencies below a frequency threshold.  
     
     
         5 . A video encoder ( 201 ) as claimed in  claim 3  wherein the means ( 211 ) for setting the encoding block size is operable to set the encoding block size to a predetermined value.  
     
     
         6 . A video encoder ( 201 ) as claimed in  claim 1  wherein the means ( 207 ,  209 ) for determining the picture region comprises means for determining the spatial frequency characteristic in response to a variance of pixel values within the picture region.  
     
     
         7 . A video encoder ( 201 ) as claimed in  claim 1  wherein the means ( 211 ) for setting the encoding block size comprises means for generating a set of allowable encoding block sizes in response to the spatial frequency characteristic; and the means ( 213 ) for encoding comprises means for selecting the encoding block size from the set of allowable encoding block sizes.  
     
     
         8 . A video encoder ( 201 ) as claimed in  claim 1  further comprising: 
 means for determining a second picture region having a second spatial frequency characteristic;    means for setting a second encoding block size for the second picture region in response to the second spatial frequency characteristic; and wherein    the means ( 213 ) for encoding the video signal is operable to encode the video signal using the second encoding block size for the second picture region.    
     
     
         9 . A video encoder ( 201 ) as claimed in  claim 1  wherein the spatial frequency characteristic comprises an indication of a degree of flatness in the picture region and the means ( 211 ) for setting the encoding block size is operable to increase the encoding block size for increasing degrees of flatness.  
     
     
         10 . A video encoder ( 201 ) as claimed in  claim 1  wherein the spatial frequency characteristic comprises an indication of a degree of uniformity in the picture region and the means ( 211 ) for setting the encoding block size is operable to increase the encoding block size for increasing degrees of uniformity.  
     
     
         11 . A video encoder ( 201 ) as claimed in  claim 1  wherein the spatial frequency characteristic comprises an indication of a concentration of energy towards lower frequencies and the means ( 211 ) for setting the encoding block size is operable to increase the encoding block size for an increasing concentration of energy towards lower frequencies.  
     
     
         12 . A video encoder ( 201 ) as claimed in  claim 1  further comprising: means for setting a quantisation level for the picture region in response to the spatial frequency characteristic; and wherein the means ( 213 ) for encoding the video signal is operable to use the quantisation level for the picture region.  
     
     
         13 . A video encoder ( 201 ) as claimed in  claim 1  wherein the video encoder ( 201 ) is a video encoder in accordance with the H.264 recommendation defined by the International Telecommunications Union.  
     
     
         14 . A video encoder ( 201 ) as claimed in  claim 13  wherein the encoding block size is selected from a set of motion estimate block sizes of inter prediction modes defined in the H.26L standard.  
     
     
         15 . A method of video encoding ( 300 ) comprising the steps of: 
 determining ( 303 ,  305 ) a picture region having a spatial frequency characteristic; 
 setting ( 307 ) an encoding block size for the picture region in response to the spatial frequency characteristic; and  
 encoding ( 309 ) the video signal using the encoding block size for the picture region.  
   
     
     
         16 . A computer program enabling the carrying out of a method according to  claim 15 .  
     
     
         17 . A record carrier comprising a computer program as claimed in  claim 16.